1. Field of Invention
This invention relates to black toner, developer containing the black toner, and a method of forming images with the developer utilizing a semiconductive magnetic brush development system. More in particular, the invention relates to black toner having specific toner particle and external additive compositions and properties such that the toner, following triboelectric contact with a carrier, exhibits a triboelectric charge of from about 35 to about 75 μC/g so as to provide a black toner image of superior image quality when used to develop electrostatic images in a semiconductive magnetic brush development system.
2. Description of Related Art
U.S. Pat. No. 5,545,501 describes an electrostatographic developer composition comprising carrier particles and toner particles with a toner particle size distribution having a volume average particle size (t) (such that 4 μm≦t≦12 μm and an average charge (absolute value) per diameter in femtocoulomb/10 μm (CT) after triboelectric contact with said carrier particles such that 1 fC/10 μm≦CT≦10 fC/10 μm characterized in that (i) said carrier particles have a saturation magnetization value, Msat, expressed in Tesla (T) such that Msat≧0.30 T, (ii) said carrier particles have a volume average particle size (Cavg) such that 30 μm≦Cavg≦60 μm, (iii) said volume based particle size distribution of said carrier particles has at least 90% of the particles having a particle diameter C such that 0.5 Cavg≦C≦2 Cavg, (iv) said volume based particles size distribution of said carrier particles comprises less than b % particles smaller than 25 μm wherein b=0.35×(Msat)2×P with Msat=saturation magnetization value, Msat, expressed in T and P=the maximal field strength of the magnetic developing pole expressed in kA/m, and (v) said carrier particles comprise a core particle coated with a resin coating in an amount (RC) such that 0.2% w/w≦RC≦2% w/w. See the Abstract. This patent describes that such developer achieves images of offset-quality in systems in which a latent image is developed with a fine hair magnetic brush. See column 4, lines 7–17 of the patent.
U.S. Pat. No. 6,319,647 describes a toner of toner particles containing at least one binder, at least one colorant, and preferably one or more external additives that is advantageously formed into a developer and used in a magnetic brush development system to achieve consistent, high quality copy images. The toner particles, following triboelectric contact with carrier particles, exhibit a charge per particle diameter (Q/D) of from 0.6 to 0.9 fC/μm and a triboelectric charge of from 20 to 25 μC/g. The toner particles preferably have an average particle diameter of from 7.8 to 8.3 microns. The toner is combined with carrier particles to achieve a developer, the carrier particles preferably having an average diameter of from 45 to 55 microns and including a core of ferrite substantially free of copper and zinc coated with a coating comprising a polyvinylidenefluoride polymer or copolymer and a polymethyl methacrylate polymer or copolymer.
U.S. Pat. No. 6,416,916 describes a toner of toner particles containing at least one binder, at least one colorant, and an external additive package comprised of zinc stearate and at least one of silicon dioxide or titanium dioxide, wherein the amount of zinc stearate is limited to about 0.10 percent by weight or less of the toner. It is reported that when the amount of zinc stearate is so limited, a developer formed from the toner exhibits excellent triboelectric charging and stability and excellent developer flow. When the developer is used in a magnetic brush development system, consistent, high quality copy images are formed substantially without any depletion defects over time.
What is still desired is a black toner for use in semiconductive magnetic brush development systems, which toner is able to develop a large number of pages per minute with substantially reduced emissions and high print quality.